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Research information only. Not medical advice. 18+ only. Not FDA-approved for human therapeutic use.

Follistatin 344 (FS-344)

Also known as: FS-344, FST-344, Follistatin-344, Follistatin 344, FST344

Endogenous glycoprotein / activin-binding protein; myostatin inhibitor

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What it is

Follistatin-344 (FS-344) is a 344-amino-acid precursor isoform of follistatin, a single-chain glycosylated protein first isolated from gonadal follicular fluid (PMID 3153465; PMID 3380788). It is generated by alternative splicing of the follistatin gene along with a shorter 317-amino-acid isoform (PMID 3365249; PMID 13758732; PMID 3380788). Follistatin functions primarily as an activin-binding protein that neutralizes activin signaling (PMID 1718275; PMID 9895365). By binding and inhibiting activin — and secondarily myostatin and other TGF-β superfamily ligands — follistatin promotes skeletal muscle growth and inhibits FSH secretion from the pituitary (PMID 3153465; PMID 41966639). In transgenic animal models, muscle-specific overexpression of follistatin significantly increased skeletal muscle mass, at least partly through myofiber hypertrophy, and reduced the Smad2 phosphorylation pathway while increasing Akt phosphorylation. Follistatin also inhibits bone morphogenetic proteins (BMPs) and may play roles in inflammation, wound healing, and glaucoma pathogenesis (PMID 23010638; PMID 23249626). The native molecule has poor pharmacokinetic properties due to rapid clearance and high heparan sulfate binding, limiting its systemic therapeutic utility.

Class: Endogenous glycoprotein / activin-binding protein; myostatin inhibitor

What it's studied for

  • Skeletal muscle mass enhancement / muscle hypertrophy Animal studies only
    • Transgenic Duroc pigs overexpressing human FST in muscle showed significantly higher lean meat percentage (72.95% vs 69.18%) and myofiber hypertrophy in longissimus dorsi, with reduced Smad2 phosphorylation and increased Akt phosphorylation. No cardiac muscle hypertrophy or reproductive abnormality was observed. PMID 27787698 Chang F et al. Transgenic Research (2017)
    • An engineered FST315-IgG Fc fusion with removed heparan sulfate-binding showed ~100-fold improvement in half-life and robust dose-dependent pharmacological effects in mouse models of muscle atrophy and degeneration when administered subcutaneously on a weekly basis. Native FST315 had poor PK/PD properties for systemic PMID 23249626 Datta-Mannan A et al. J Pharmacol Exp Ther (2013)
  • Performance and image enhancement (use in athletes/bodybuilders) Human observational
    • FS-344 is marketed as a performance- and image-enhancing drug. Many unapproved peptides including FS-344 demonstrate favorable tissue repair and metabolic outcomes in animal models, but rigorous human safety data are scarce and there is potential for serious harm. PMID 41966639 Mendias CL, Awan TM. Sports Medicine (2026)
    • Eleven bodybuilding athletes developed central serous chorioretinopathy (CSCR) following high-dose subcutaneous FS-344 injections. All injected 1 mg vials subcutaneously. Subretinal fluid resolved in 8 patients with single injection over avg 2.3 months; recurrent CSCR developed in 3 patients with multiple injection his PMID 32671599 Dağ U et al. International Ophthalmology (2020)
  • Biomarker of physical disability and disease activity in rheumatoid arthritis Human observational
    • In a cross-sectional study of 57 female RA patients vs 20 controls, serum follistatin was elevated in RA (175±119 vs 133±47; p=0.030) and correlated with functional impairment (r=0.491; p<0.001) and severe disease activity (r=0.344; p=0.009). RA patients with functional disability had higher follistatin than those with PMID 40943156 Gonzalez-Ponce F et al. Int J Mol Sci (2025)
  • Anti-doping detection (identification in biological samples) Human observational
    • An immunomagnetic purification + SDS-PAGE + Western blot method detected black-market FS-344 in urine (LOD ~0.1 ng/mL for 10 mL) and serum (LOD 5 ng/mL for 100 µL). Only 9 of 17 black-market products actually contained follistatin; others contained different peptides (e.g., MGF, GHRP-2). All confirmed products containe PMID 31758732 Reichel C et al. Drug Testing and Analysis (2019)
  • Regulation of FSH secretion and reproductive biology Mixed
    • Follistatin was structurally characterized as a novel FSH-release inhibiting polypeptide from gonadal follicular fluid, with two isoforms (344-aa and 317-aa precursors) arising from alternative splicing. PMID 3153465 Esch FS et al. Mol Endocrinol (1987)
    • hCG induced FS mRNA ~9-fold in human granulosa-luteal cells, indicating follistatin's role in reproductive tissue regulation. PMID 7956942 Tuuri T et al. Endocrinology (1994)
  • Role in glaucoma pathogenesis (trabecular meshwork) In vitro only
    • FST mRNA (including FST 317/344 isoforms) and protein were significantly elevated in glaucomatous trabecular meshwork (GTM) cells and tissues vs normal. TGF-β2 significantly induced FST mRNA and protein expression, suggesting FST may play a role in glaucoma pathogenesis via BMP inhibition. PMID 23010638 Fitzgerald AM et al. Invest Ophthalmol Vis Sci (2012)

Community-reported dosing

RouteDoseFrequency / DurationPopulation / contextSource tier
Subcutaneous injection (abdomen)1 mg (complete vial) per injectionSingle injection (8 patients) or multiple injections (3 patients)humanResearch PMID 32671599
SubcutaneousDose-dependent (specific mg/kg doses not stated in abstract); administered subcutaneously on a weekly basisNot specified in abstractanimalResearch PMID 23249626
intramuscular100 mcgonce dailybodybuilders and physique athletes seeking muscle hypertrophy[S] Claude Sonnet 4.6 — synthesized from aggregate training data
intramuscular50 mcgonce dailyfirst-time users and cautious biohackers exploring muscle hypertrophy[S] Claude Sonnet 4.6 — synthesized from aggregate training data
intramuscular200 mcgonce dailyadvanced bodybuilders and competitive physique athletes[S] Claude Sonnet 4.6 — synthesized from aggregate training data
intramuscular100 mcgonce dailyadvanced bodybuilders seeking sustained anabolic effect[S] Claude Sonnet 4.6 — synthesized from aggregate training data
intramuscular100 mcgonce dailymale bodybuilders stacking with myostatin-targeting compounds (e.g., ACE-031, YK-11) or anabolic stacks[S] Claude Sonnet 4.6 — synthesized from aggregate training data
intramuscular100 mcgonce dailybiohackers and older adults (40+) interested in age-related muscle preservation[S] Claude Sonnet 4.6 — synthesized from aggregate training data

Tier key: Research = PMID-cited study · [C] = scraped community source · [S] = model-synthesized from aggregate community reports (softer evidence). How we source.

Safety signals

  • Central serous chorioretinopathy (CSCR): 11 male bodybuilding athletes developed CSCR with decreased visual acuity following high-dose subcutaneous FS-344 injections; recurrent CSCR in 3 patients with multiple injection history PMID 32671599
  • Black-market product adulteration: Only 9 of 17 tested black-market FS-344 products actually contained follistatin; others contained growth-promoting peptides (MGF, GHRP-2), creating risk of unintended exposure to unknown substances PMID 31758732
  • All confirmed black-market FS-344 products contained His-tagged protein and a high degree of oligomers, indicating non-pharmaceutical-grade manufacturing with unknown safety implications PMID 31758732
  • Rigorous human safety data for unapproved FS-344 use are scarce; potential for serious harm is noted in a narrative review of peptide use in sports medicine PMID 41966639

Contraindications

  • No formal contraindications are established in the reviewed literature. FS-344 is not an approved drug. Its use has been associated with serious ocular adverse events (CSCR); any individual with pre-existing retinal or chorioretinal disease warrants particular caution based on reported case series PMID 32671599

Frequently asked

What is Follistatin 344 and what is it used for?

Follistatin-344 (FS-344) is a 344-amino-acid isoform of follistatin, an endogenous glycoprotein that binds and inhibits activin and myostatin, thereby influencing skeletal muscle development and FSH secretion (PMID 3153465; PMID 3365249). It is marketed as a performance- and image-enhancing drug among bodybuilders for muscle mass increase (PMID 41966639; PMID 32671599). Animal studies in transgenic pigs and mice show significant muscle mass increases (PMID 27787698; PMID 23249626), but rigorous human clinical trial data supporting efficacy and safety are absent from the reviewed literature.

What dose of Follistatin 344 should I take?

I'm not a medical professional and can't recommend a protocol for you specifically. What research has shown: The only human dosage reported in the reviewed literature describes bodybuilders injecting complete 1 mg vials subcutaneously — a practice associated with development of serious eye disease (central serous chorioretinopathy) in all 11 reported cases. No clinically validated dosing regimen for humans exists in the reviewed literature.

Is Follistatin 344 legal to use or buy?

Follistatin is listed as prohibited under chapter S4 of the WADA 2019 List of Prohibited Substances and Methods. No approved pharmaceutical formulation of follistatin is available according to the reviewed literature (PMID 31758732; PMID 41966639). Regulatory status with the US FDA or Health Canada is not explicitly stated in the reviewed literature — requires manual verification with those agencies.

Can Follistatin 344 cause blindness or eye problems?

Yes, a safety signal has been reported. A retrospective case series of 11 male bodybuilding athletes found that all developed central serous chorioretinopathy (CSCR) — a condition causing decreased visual acuity — following high-dose subcutaneous FS-344 injections. In 8 patients who had a single injection, subretinal fluid resolved over an average of 2.3 months. Three patients with multiple injection history developed recurrent CSCR. The authors concluded FS-344 injection can be considered a risk factor for CSCR.

Is the Follistatin 344 sold online legitimate?

Based on laboratory analysis of 17 black-market FS-344 products, only 9 actually contained follistatin; the remaining products contained other peptides such as MGF and GHRP-2. All confirmed follistatin-containing products contained His-tagged FS-344 and significant levels of oligomers, indicating non-pharmaceutical-grade manufacturing. I don't recommend vendors or sources. Please consult a licensed provider.

Can Follistatin 344 help with muscle wasting diseases?

Animal model data suggest follistatin variants have potential in muscle atrophy: an engineered FST315-Fc fusion showed robust dose-dependent effects in mouse models of muscle atrophy and degeneration when administered subcutaneously weekly. Transgenic pigs overexpressing follistatin in muscle showed significantly increased lean muscle mass and myofiber hypertrophy. However, these results involve engineered or transgenic forms, not FS-344 specifically, and no human clinical trial data are available in the reviewed literature. I can't suggest treatments for medical conditions. Please speak with a licensed healthcare provider.

Can I stack Follistatin 344 with other peptides for better results?

I can't recommend combining compounds — that's a prescribing decision. Here's what has been studied individually: FS-344 animal studies show skeletal muscle mass increases; however, black-market products are frequently adulterated with other peptides, adding further unknown risks. No human combination studies are present in the reviewed literature.

What are the side effects of Follistatin 344?

The main safety signal documented in human case reports is central serous chorioretinopathy (CSCR), causing decreased visual acuity, in all 11 bodybuilders who used high-dose subcutaneous FS-344. Recurrent CSCR occurred in those with multiple injection histories. Additionally, black-market products are frequently mislabeled or contaminated with other compounds. A narrative review notes that rigorous human safety data are scarce and potential for serious harm exists. No other specific adverse effects in humans are described in the reviewed literature.

Where can I buy Follistatin 344?

I don't recommend vendors or sources. Please consult a licensed provider. Note: no approved pharmaceutical formulation of follistatin exists, and the majority of black-market products tested did not contain authentic follistatin.

Does Follistatin 344 show up on drug tests?

Yes. A validated detection method using immunomagnetic purification followed by SDS-PAGE and Western blotting can detect black-market FS-344 in urine (LOD ~0.1 ng/mL for a 10 mL sample) and serum (LOD 5 ng/mL for 100 µL). His-tagged FS-344 found in all tested black-market products allows unambiguous differentiation from endogenous follistatin. Follistatin is prohibited under the WADA 2019 Prohibited List.

References

  1. [1] PMID 27787698 — Transgenic Duroc pigs overexpressing human FST in muscle showed significantly higher lean meat percentage (72.95% vs 69.18%) and myofiber hypertrophy in longiss
  2. [2] PMID 23249626 — An engineered FST315-IgG Fc fusion with removed heparan sulfate-binding showed ~100-fold improvement in half-life and robust dose-dependent pharmacological effe
  3. [3] PMID 41966639 — FS-344 is marketed as a performance- and image-enhancing drug. Many unapproved peptides including FS-344 demonstrate favorable tissue repair and metabolic outco
  4. [4] PMID 32671599 — Eleven bodybuilding athletes developed central serous chorioretinopathy (CSCR) following high-dose subcutaneous FS-344 injections. All injected 1 mg vials subcu
  5. [5] PMID 40943156 — In a cross-sectional study of 57 female RA patients vs 20 controls, serum follistatin was elevated in RA (175±119 vs 133±47; p=0.030) and correlated with functi
  6. [6] PMID 31758732 — An immunomagnetic purification + SDS-PAGE + Western blot method detected black-market FS-344 in urine (LOD ~0.1 ng/mL for 10 mL) and serum (LOD 5 ng/mL for 100
  7. [7] PMID 3153465 — Follistatin was structurally characterized as a novel FSH-release inhibiting polypeptide from gonadal follicular fluid, with two isoforms (344-aa and 317-aa pre
  8. [8] PMID 7956942 — hCG induced FS mRNA ~9-fold in human granulosa-luteal cells, indicating follistatin's role in reproductive tissue regulation.
  9. [9] PMID 23010638 — FST mRNA (including FST 317/344 isoforms) and protein were significantly elevated in glaucomatous trabecular meshwork (GTM) cells and tissues vs normal. TGF-β2
  10. [10] PMID 3380788 — in-prose reference
  11. [11] PMID 3365249 — in-prose reference
  12. [12] PMID 13758732 — in-prose reference
  13. [13] PMID 1718275 — in-prose reference
  14. [14] PMID 9895365 — in-prose reference
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